This argument reminds me of the guy who blames his rifle because he missed a deer at 50 yards. I got news for him,..... it ain't the rifle.
I have no experience with any of the detectors mentioned in this thread except the 800. But after 1000s of hours hunting mostly old parks and curbstrips, with an occasional yard or empy lot thown in, I have come to the conclusion the the 800 misses nothing that is detectable within its individual settings. However, I as the detectorist have missed a lot, either due to not understanding what I was hearing, or not getting the coil over the target.
In one particular park, I started out finding very little with the 800, but as I gained experience , I began finding more. Hunting over the same areas, but gridding in different directions, digging the " iffy " signals and putting up with bucketloads of falsing Iron, pulltabs and bottlecaps, I was able to find even more good stuff. Eventually, good targets became more and more rare. And just when I was coming to the conclusion there was nothing good left to find, I found my first and only seated coin --in an area I had been over at least a half dozen times-- and it was not even that deep. I could say the detector missed it before, but if I am being honest, it wasnt the detector. It was the detectorist that missed it.
Most of the people on here know a hell of alot more than I do about the technical aspects of many differet detectors. And I have to trust that the engineers at the detector companies know even more. But let's be truthful here. All of the modern detectors will detect metal. Some have a bit more depth capability , some are tuned to different levels of conductitvity, some handle mineralization better than others, but I don't believe any of them "miss" detectable targets if they are properly set up for the conditions.